Hi.
Thanks so much for this help. I managed to delete the offending certificate with help from this link https://github.com/duplicati/duplicati/issues/4650
On the ASUSTOR NAS using SSH
Open the file ca-certificates.crt
/etc/ssl/certs # vi ca-certificates.crt
The problem was that it does not give you the certificate name before the BEGIN CERTIFICATE so i had to search for the actual certificate using parts of the actual certificate and checking that it matched with the one in the post.
I then deleted this from the file, saved it and then re run
sudo cert-sync /etc/ssl/certs/ca-certificates.crt
This told me that the DST ROOT CA X3 Cert was removed.
# sudo cert-sync /etc/ssl/certs/ca-certificates.crt
Mono Certificate Store Sync - version 6.12.0.107
Populate Mono certificate store from a concatenated list of certificates.
Copyright 2002, 2003 Motus Technologies. Copyright 2004-2008 Novell. BSD licensed.
Importing into legacy system store:
I already trust 127, your new list has 126
1 previously trusted certificates were removed.
Certificate removed: O=Digital Signature Trust Co., CN=DST Root CA X3
Import process completed.
Importing into BTLS system store:
I already trust 127, your new list has 126
1 previously trusted certificates were removed.
Certificate removed: O=Digital Signature Trust Co., CN=DST Root CA X3
Import process completed.
I restarted Duplicati & Mono in my NAS and then ran my backup job
So far so good, it runs with no error.
The update also now seems to work!
Thank you again so much for your help.